(Bloomberg Gadfly) -- Even by recent standards, February hasbeen an eventful month for health insurance.

Last week brought thwarted mergers, threats by insurers toleave the Affordable Care Act's individual exchanges, andthe release of a (very) rough sketch of a possible GOPrepeal-and-replace plan for the ACA.

Related: 10 influencers on ACA repealefforts

What's missing in that skeletal outline is how to pay for newinitiatives, such as an expanded tax credits to help people buyinsurance, while also repealing the new taxes established by theACA.

Some in the GOP are floating one possiblesolution: capping the federal tax breaks workers and companiesget for employer-provided health insurance.

The tax code's exclusion on health benefits is the nearly80-year-old foundation of America's employer-based insurancesystem. The idea of capping it is a potentially scary prospect forinsurers, raising questions about the future of the employermarket, through which about 150 million Americans getinsurance.

Economists like the idea of getting rid of the exclusion. It'sthe single largest American tax expenditure and is responsible formany of the oddities, distortions, and inequities of employer-basedAmerican health care. The exclusion costs the government more than$250 billion a year, and GOP leadership appears to be seriouslyconsidering a cap.

But however compelling their arguments are, economists and GOPsympathizers are substantially outnumbered in thisfight. Employees really like getting generousbenefits without being exposed to the full cost. And companies getto provide a highly popular benefit without paying taxes on it,effectively paying employees extra without tax consequences.

This would effectively be a large tax hike on corporationsand higher-income working people -- not exactly traditionalRepublican targets. The lobbying against it has alreadycommenced.

Capping this tax exclusion and removing the ACA's employerinsurance mandate would be a two-fold blow to the employer market.Businesses will likely be less inclined to provide insurance. Andtheir employees may be less inclined to get insurance through workif they're exposed to higher costs.

Related: ACA reform: How to keep calm and carryon

That would be a negative for insurers -- such as UnitedHealthGroup Inc., Aetna Inc., Cigna Corp., and Anthem Inc. -- that focusin large part on employers. They will have to adapt to amuch-changed market that may take years to mature.

These insurers may be able to scoop up lost enrollees in aTrumpcare version of the ACA's individual market. But employers areoften able to provide generous benefits as a result of the taxexclusion, while the individual market may trend towardsless-expensive plans that offer less coverage. The tax creditsproposed in the GOP outline are tied to age rather than income, andmay render comprehensive coverage unaffordable forsome.

The GOP proposal would encourage young and healthy patients tobuy cheaper insurance plans, leaving only older and sicker patientson employer health plans. That would significantly increase thecost to insurers -- and remember, it was cost overruns that ledsome insurers to withdraw from the ACA's individual exchanges.

As noisy as the tussle over the ACA and individualexchanges has been, it concerns a relatively small portion of themarket. The GOP's proposed shift in tax policy would have afar bigger impact on insurers and employers.

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